A high-sensitive X-ray computed tomography (CT) system is useful for decreasing absorbed dose for patients, and a
dark-count-less photon-counting CT system was developed. X-ray photons are detected using a YAP(Ce) [cerium-doped
yttrium aluminum perovskite] single crystal scintillator and an MPPC (multipixel photon counter). Photocurrents are
amplified by a high-speed current-voltage amplifier, and smooth event pulses from an integrator are sent to a high-speed comparator. Then, logical pulses are produced from the comparator and are counted by a counter card. Tomography is accomplished by repeated linear scans and rotations of an object, and projection curves of the object are obtained by the linear scan. The image contrast of gadolinium medium slightly fell with increase in lower-level voltage (Vl) of the comparator. The dark count rate was 0 cps, and the count rate for the CT was approximately 250 kcps.